1. Field of the Invention
The present invention relates to a novel photosensitive resin which can be developed in water and which can be used as a photosensitive resin composition. More specifically, the present invention relates to a composition which is useful as a photosensitive soldering resist for printed circuit board.
2. Discussion of the Background
Soldering resists are used to prevent troubles when soldering parts onto a printed circuit board and to protect the circuit board having parts mounted thereon for a long period of time. However, in recent years, as the size of electronic appliances has decreased, the density of the printed circuit board has increased as parts are mounted on the surface. To allow for the higher density of the printed circuit board, the resist has been changed from a screen-printing-type resist to a photographic printing-type resist.
Alkaline aqueous solutions are mainly used for developing photographic-printing-type resists because they improve the working environment and protect the environment. A photosensitive soldering resist which can be developed in alkali is described in Japanese Patent Publication No. 54,390/1989. Therein, a photo-setting or thermosetting liquid resist ink composition comprising an actinic light-curable resin obtained by reacting a reaction product of a novolak type epoxy compound and an unsaturated monocarboxylic acid with a saturated or unsaturated polybasic acid anhydride, a photopolymerization initiator, a diluent and an epoxy compound is disclosed. Although the properties as the soldering resist are satisfactory, when the coating is conducted through spray coating or curtain coating, dilution with a large amount of an organic solvent is required, posing problems such as environmental pollution, ignition and the like. Accordingly, an improvement has been in high demand.
Japanese Laid-Open Patent Application (Kokai) Nos. 120,308/1990 and 157,965/1994 disclose a soldering resist ink composition which is obtained by using an alicyclic epoxy compound as a starting material and which can be developed with an alkaline aqueous solution. However, since this composition is developed with a dilute alkaline aqueous solution, it is necessary not only to prepare a developer and control a concentration thereof but also to neutralize the fatigue developer. Further, the alkali ion component of the developer remains in the resist coated film, which seems to pose a problem with electrical characteristics. For this reason, it is indispensable to conduct a water-washing step after the development step.
In order to solve these problems, liquid photosensitive soldering resists which can be diluted in water and/or can be developed in water have been proposed.
For example, Japanese Laid-Open Patent Application (Kokai) No. 294,352 discloses a photosensitive resin composition which is formed by reacting an aromatic epoxy resin with an unsaturated monocarboxylic acid, then reacting the reaction mixture with an unsaturated polybasic acid anhydride, and then neutralizing the reaction product with amine, a thermosetting compound, a photopolymerization initiator and a reactive diluent. Japanese Laid-Open Patent Application (Kokai) No. 294,354/1992 discloses a photosensitive oligomer obtained by neutralizing with amine a reaction product of a copolymer of a (meth)acrylate ester and (meth)acrylic acid with an epoxy group-containing (meth)acrylate ester, a photopolymerization initiator and a reactive diluent. These compositions can be diluted with water, but involve problems in that a dilute alkaline aqueous solution is required in the development, the amine used in the neutralization is vaporized in the drying step used to obtain a tack-free film, and decreased developing occurs in this step.
U.S. Pat. Nos. 3,697,398, 3,936,405 and 3,962,165 disclose a cationic electrodeposition coating comprising an epoxy resin and an onium compound such as a quaternary ammonium salt, a tertiary sulfonium salt and a quaternary phosphonium salt.
U.S. Pat. No. 4,338,232 discloses epoxy resin derivatives containing a photosensitive group and a tertiary sulfonium salt. Japanese Laid-Open Patent Application (Kokai) No. 1,858/1990 discloses that analogous aromatic epoxy resin derivatives are used in a photosensitive soldering resist which can be diluted and developed in water. In such aromatic epoxy resins, a phenolic hydroxyl group of a phenolic novolak resin or a cresol novolak resin is reacted with epichlorohydrin under alkaline conditions, so that impurities such as sodium and chlorine tend to remain in the epoxy resin product.
When aromatic epoxy resin derivatives are used as main components in a photosensitive soldering resist, the aromatic ring absorbs ultraviolet light or the like used in the exposure step. When this occurs, permeation of actinic light is poor, and the energy is not effectively utilized in the photo-setting. Accordingly, an exposure amount of more than approximately 300 mJ/cm.sup.2 is required in a photosensitive soldering resist. A photosensitive soldering resist which can be used in a printed circuit board having a fine pattern is required to have excellent heat resistance, excellent resistance to plating and high levels of electrical characteristics. None of existing products have satisfied these requirements.